1. Technical Field
The present invention relates to a variable capacitor, and in particular to a variable capacitor employing MEMS (Micro Electro Mechanical System) technology.
2. Related Art
Recently there has been significant progress in wireless communication equipment, typified by mobile phones. The next generation of mobile phones will use multiple frequency bands. This has lead to progress in the development of multi-frequency receivable RF (Radio Frequency) front ends. The problem with this is the increase in the number of components. In particular it is difficult to make common passive components such as capacitors and inductors, and so the number of components increases along with the increase in the number of frequency bands. As the number of components increases problems arise in that the volume of the RF front end becomes large and there the const becomes expensive.
MEMS technology is drawing attention as a technology to solve such problems, with the ability to make components dramatically more compact. MEMS technology forms micro-scale mechanical structures, and produces ultra small devices. For example, a MEMS tunable capacitor can be produced capable of having plural impedances in a single device.
A tunable matching circuit can be configured using a MEMS tunable capacitor. By applying such a tunable matching circuit to a RF front end, it becomes possible to automatically reconfigure the circuit itself for each of the frequencies and each system, with the expectation that the number of antennae and number of individual amplifiers can be dramatically reduced. Various MEMS tunable capacitors are therefore proposed.
For example, in Japanese Patent Application Laid-Open (JP-A) No. 2006-261480, a MEMS variable capacitor is described in which a first electrode and a second electrode are disposed fixedly on a substrate, a moveable electrode is moved by an actuator with the application of a voltage, and the electric capacitance between the first electrode and the second electrode is changed by contact with the moveable electrode.
In JP-A No. 2005-224934, a ultra small movable structure (MEMS) is described in which a first electrode and a second electrode are disposed facing each other in an up-down direction. Driving force is generated in this ultra small movable structure by applying a voltage across the first electrode and the second electrode, due to electrostatic force. The surface area over which electrostatic force acts is changed by the second electrode, which is a moveable electrode, moving in the up-down direction, changing the electrostatic capacitance.
However, a battery of about 3V is used in a small scale wireless communication device such as a mobile phone. In order to be applicable to mobile phones or the like, a MEMS tunable capacitor is therefore required capable of low voltage operation with an operating voltage of 3V or less. Namely, the electrostatic capacitance must be capable of being changed by a large amount, within the range of 0V to 3V In cases where comb electrodes are used, like that described in JP-A No. 2005-224934, there is the merit that the surface area over which electrostatic force acts becomes large, and the breadth of change in the electrostatic capacitance can be made large.
In Japanese Patent Application Publication No. 2006-518926, in a similar manner to in JP-A No. 2005-224934, a electromechanical micro variable capacitance diode (MEMS) is described that applies comb electrodes to a fixed electrode and a moveable electrode. In this MEMS variable capacitance diode, the electrostatic capacitance is changed by applying a voltage across the fixed electrode and the moveable electrode, and the moveable electrode moving due to electrostatic force. Comb electrodes are also employed in other MEMS devices, such as in mirror elements and the like. For example, JP-A No. 2005-341788 describes a micro-actuator that moves a movable portion in a substrate base direction by the action of a electrostatic force between a first comb electrode of the substrate base and a second comb electrode of movable portion due to application of a voltage.
As stated above, for application to mobile phones or the like, MEMS tunable capacitors (variable capacitors), using comb electrodes and capable of low voltage operation with an operating voltage of 3V or less, are being researched into. However, a problem arises in that these variable capacitors are not capable of stably maintaining the position of the moveable electrode after movement. The moveable electrode is maintained in a specific position by electrostatic force. The electrostatic capacitance changes if the position of the moveable electrode changes due to external vibration, leading to problems of unstable operation.
The fixed electrode and the moveable electrode operate by comb teeth portions thereof meshing together with each other. Consequently, there is concern of short circuiting occurring if external vibration is imparted during operation and the fixed electrode and the moveable electrode make contact with each other.